Synthesis of Tris(pyrazolyl)borate-Stabilized Vinyl, Allyl, and Homoallyl Tungsten Oxides and Their Selective Oxyfunctionalization by Singlet Oxygen and Dioxirane

Abstract

The synthesis of new tris(pyrazolyl)borato-substituted alkenyltungsten complexes and their oxyfunctionalization was investigated. The derivatives 2a−e of the title compounds were prepared in 40−70% yield by Grignard reaction of [Tp*W(O)2Cl] (1) [Tp* = hydridotris(3,5-dimethyl-1-pyrazolyl)borato] and subsequent treatment with molecular oxygen. These alkenyltungsten complexes, except the homoallyl complex 2b with a monosubstituted double bond, displayed a high reactivity toward singlet oxygen (1O2) to result in the corresponding allylic hydroperoxides by the Schenk ene reaction. While in the photooxygenation of the homoallyl complex 2a no special influence of the [Tp*W(O)2] fragment was observed, the allyl complexes 2c,d afforded stereoselectively the Z-configured products. Stereocontrol derives presumably from hyperconjugative stabilization by the W−C bond (β effect) of the perepoxide-like transition state. The allylic hydroperoxide, derived from the 1O2 ene reaction of the vinyl complex 2e, led by Hock-ty...